Network Working Group Jonathan Trostle
INTERNET-DRAFT Cisco Systems
Category: Standards Track Mike Swift
University of WA
John Brezak
Microsoft
Bill Gossman
Cisco Systems
Kerberos Set/Change Password: Version 2
<draft-ietf-cat-kerberos-set-passwd-06.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [6].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet- Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This draft expires on December 31st, 2001. Please send comments to
the authors.
1. Abstract
This proposal specifies a Kerberos (RFC 1510 [3]) change/set password
protocol and a Kerberos change/set key protocol. The protocol
consists of a single request and reply message. The request message
includes both AP-REQ and KRB-PRIV submessages; the new password is
contained in the KRB-PRIV submessage which is encrypted in the
subsession key from the AP-REQ. The original Kerberos change password
protocol did not allow for an administrator to set a password for a
new user. This functionality is useful in some environments, and this
proposal allows password setting as well as password changing. The
protocol includes fields in the request message to indicate the
principal which is having its password set. We also extend the
set/change protocol to allow a client to send a sequence of keys to
Trostle, Swift, Brezak, Gossman [Page 1]
INTERNET DRAFT June 2001 Expires December 2001
the KDC instead of a cleartext password. If in the cleartext password
case, the cleartext password fails to satisfy password policy, the
server should use the result code KRB5_KPASSWD_POLICY_REJECT.
2. Conventions used in this document
The key words "MUST", "MUSTNOT", "REQUIRED", "SHALL", "SHALLNOT",
"SHOULD", "SHOULDNOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC2119 [7].
3. Definitions from RFC 1510
We include some of the relevant ASN.1 definitions from RFC 1510 in
this section.
Realm ::= GeneralString
PrincipalName ::= SEQUENCE {
name-type[0] INTEGER,
name-string[1] SEQUENCE OF GeneralString
}
KerberosTime ::= GeneralizedTime
-- Specifying UTC time zone (Z)
HostAddress ::= SEQUENCE {
addr-type[0] INTEGER,
address[1] OCTET STRING
}
EncryptedData ::= SEQUENCE {
etype[0] INTEGER, -- EncryptionType
kvno[1] INTEGER OPTIONAL,
cipher[2] OCTET STRING -- ciphertext
}
EncryptionKey ::= SEQUENCE {
keytype[0] INTEGER,
keyvalue[1] OCTET STRING
}
Checksum ::= SEQUENCE {
cksumtype[0] INTEGER,
checksum[1] OCTET STRING
}
AP-REQ ::= [APPLICATION 14] SEQUENCE {
pvno [0] INTEGER, -- indicates Version 5
msg-type [1] INTEGER, -- indicates KRB_AP_REQ
ap-options[2] APOptions,
ticket[3] Ticket,
authenticator[4] EncryptedData
}
Trostle, Swift, Brezak, Gossman [Page 2]
INTERNET DRAFT June 2001 Expires December 2001
APOptions ::= BIT STRING {
reserved (0),
use-session-key (1),
mutual-required (2)
}
Ticket ::= [APPLICATION 1] SEQUENCE {
tkt-vno [0] INTEGER, -- indicates Version 5
realm [1] Realm,
sname [2] PrincipalName,
enc-part [3] EncryptedData
}
-- Encrypted part of ticket
EncTicketPart ::= [APPLICATION 3] SEQUENCE {
flags[0] TicketFlags,
key[1] EncryptionKey,
crealm[2] Realm,
cname[3] PrincipalName,
transited[4] TransitedEncoding,
authtime[5] KerberosTime,
starttime[6] KerberosTime OPTIONAL,
endtime[7] KerberosTime,
renew-till[8] KerberosTime OPTIONAL,
caddr[9] HostAddresses OPTIONAL,
authorization-data[10] AuthorizationData OPTIONAL
}
-- Unencrypted authenticator
Authenticator ::= [APPLICATION 2] SEQUENCE {
authenticator-vno[0] INTEGER,
crealm[1] Realm,
cname[2] PrincipalName,
cksum[3] Checksum OPTIONAL,
cusec[4] INTEGER,
ctime[5] KerberosTime,
subkey[6] EncryptionKey OPTIONAL,
seq-number[7] INTEGER OPTIONAL,
authorization-data[8] AuthorizationData OPTIONAL
}
AP-REP ::= [APPLICATION 15] SEQUENCE {
pvno [0] INTEGER, -- represents Kerberos V5
msg-type [1] INTEGER, -- represents KRB_AP_REP
enc-part [2] EncryptedData
}
EncAPRepPart ::= [APPLICATION 27] SEQUENCE {
ctime [0] KerberosTime,
cusec [1] INTEGER,
subkey [2] EncryptionKey OPTIONAL,
seq-number [3] INTEGER OPTIONAL
Trostle, Swift, Brezak, Gossman [Page 3]
INTERNET DRAFT June 2001 Expires December 2001
}
Here is the syntax of the KRB-ERROR message:
KRB-ERROR ::= [APPLICATION 30] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
ctime[2] KerberosTime OPTIONAL,
cusec[3] INTEGER OPTIONAL,
stime[4] KerberosTime,
susec[5] INTEGER,
error-code[6] INTEGER,
crealm[7] Realm OPTIONAL,
cname[8] PrincipalName OPTIONAL,
realm[9] Realm, -- Correct realm
sname[10] PrincipalName, -- Correct name
e-text[11] GeneralString OPTIONAL,
e-data[12] OCTET STRING OPTIONAL
}
The KRB-PRIV message is used to send the request and reply data:
KRB-PRIV ::= [APPLICATION 21] SEQUENCE {
pvno[0] INTEGER,
msg-type[1] INTEGER,
enc-part[3] EncryptedData
}
EncKrbPrivPart ::= [APPLICATION 28] SEQUENCE {
user-data[0] OCTET STRING,
timestamp[1] KerberosTime OPTIONAL,
usec[2] INTEGER OPTIONAL,
seq-number[3] INTEGER OPTIONAL,
s-address[4] HostAddress,
-- sender's addr
r-address[5] HostAddress OPTIONAL
-- recip's addr
}
4. The Protocol
The service SHOULD accept requests on UDP port 464 and TCP port 464
as well. Use of other ports can significantly increase the complexity
and size of IPSEC policy rulesets in organizations that have IPSEC
capable nodes.
The protocol consists of a single request message followed by a
single reply message. For UDP transport, each message must be fully
contained in a single UDP packet.
For TCP transport, there is a 4 octet header in network byte order
that precedes the message and specifies the length of the message.
This requirement is consistent with the TCP transport header in
Trostle, Swift, Brezak, Gossman [Page 4]
INTERNET DRAFT June 2001 Expires December 2001
1510bis.
Request Message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message length | protocol version number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AP-REQ length | AP-REQ data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ KRB-PRIV message /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All 16 bit fields are in network byte order.
message length field: contains the number of bytes in the message
including this field.
protocol version number: contains the hex constant 0x0002 (network
byte order).
AP-REQ length: length of AP-REQ data, in bytes. If the length is
zero, then the last field contains a KRB-ERROR message instead of a
KRB-PRIV message.
AP-REQ data: (see [3]) For a change password/key request, the AP-REQ
message service ticket sname, srealm principal identifier is
kadmin/changepw@REALM where REALM is the realm of the change password
service. The same applies to a set password/key request except the
principal identifier is kadmin/setpw@REALM. The authenticator in the
AP-REQ MUST contain a subsession key (which will be used to encrypt
the KRB-PRIV user data field - see below). The KDC may have stronger
pseudorandom generating capability then the clients; thus, the client
SHOULD use the session key as an input (along with additional locally
pseudorandom generated bits) into the generation of the subsession
key. To enable setting of passwords/keys, it is not required that the
initial flag be set in the Kerberos service ticket. The initial flag
is required for change requests, but not for set requests. We have
the following definitions:
old passwd initial flag target principal can be
in request? required? distinct from
authenticating principal?
change password: yes yes no
set password: no policy (*) yes
set key: no policy (*) yes
change key: no yes no
Trostle, Swift, Brezak, Gossman [Page 5]
INTERNET DRAFT June 2001 Expires December 2001
policy (*): implementations SHOULD allow administrators to set the
initial flag required for set requests policy to either yes or no.
Clients MUST be able to retry set requests that fail due to error 7
(initial flag required) with an initial ticket. Clients SHOULDNOT
cache service tickets targetted at kadmin/changepw.
KRB-PRIV message (see [3]) This KRB-PRIV message must be encrypted
using the subsession key from the authenticator in the AP-REQ. The
authenticator MUST contain a subsession key. The timestamp and usec
fields of the KRB-PRIV message MUST be present, and the data values
MUST be copies of the same data values from the authenticator. The
recipient should ignore the sender address field in the KRB-PRIV
message.
The user-data component of the message contains the DER encoding of
the ChangePasswdData ASN.1 type described below:
ChangePasswdData ::= SEQUENCE {
passwds [0] PasswordSequence OPTIONAL,
keys [1] KeySequences OPTIONAL,
-- exactly one of the above two will be
-- present, else KRB5_KPASSWD_MALFORMED
-- error will be returned by the server.
targname[2] PrincipalName OPTIONAL,
-- only present in set password/key: the
-- principal which will have its password
-- or keys set. Not present in a set request
-- if the client principal from the ticket is
-- the principal having its passwords or keys
-- set.
targrealm[3] Realm OPTIONAL,
-- only present in set password/key: the realm
-- for the principal which will have its
-- password or keys set. Not present in a set
-- request if the client principal from the
-- ticket is the principal having its
-- passwords or keys set.
flags[4] RequestFlags OPTIONAL
-- 32 bit string
}
KeySequences ::= SEQUENCE (SIZE (1..MAX)) OF KeySequence
KeySequence ::= SEQUENCE {
key[0] EncryptionKey,
salt[1] OCTET STRING OPTIONAL,
-- depends on enc type, not currently used
salt-type[2] INTEGER OPTIONAL
-- depends on enc type, not currently used
}
PasswordSequence ::= SEQUENCE {
newpasswd[0] OCTET STRING,
oldpasswd[1] OCTET STRING OPTIONAL
Trostle, Swift, Brezak, Gossman [Page 6]
INTERNET DRAFT June 2001 Expires December 2001
-- oldpasswd always present for change
-- password but not present for set
-- password, set key, or change key
-- NOTE: the passwords are UTF8 strings.
}
RequestFlags ::= BIT STRING (SIZE (32..MAX))
-- reserved(0)
-- request-srv-gen-keys(1)
-- only in change/set keys
-- if the client desires
-- server to contribute to
-- keys;
-- server will return keys
The server must verify the AP-REQ message, check whether the client
principal in the ticket is authorized to set/change the password/keys
(either for that principal, or for the principal in the targname
field if present), and decrypt the new password/keys. The server also
checks whether the initial flag is required for this request,
replying with status 0x0007 if it is not set and should be. An
authorization failure is cause to respond with status 0x0005. For
forward compatibility, the server should be prepared to ignore fields
after targrealm in the structure that it does not understand.
If the passwds field is present, it contains the new cleartext
password (with the old cleartext password for a change password
operation). Otherwise the keys field is present, and it contains a
sequence of encryption keys.
In the cleartext password case, if the old password is sent in the
request, the request MUST be a change password request. If the old
password is not present in the request, the request MUST be a set
password request. The server should apply policy checks to the old
and new password after verifying that the old password is valid. The
server can check validity by obtaining a key from the old password
with a keytype that is present in the KDC database for the user and
comparing the keys for equality. The server then generates the
appropriate keytypes from the password and stores them in the KDC
database. If all goes well, status 0x0000 is returned to the client
in the reply message (see below). For a change password operation,
the initial flag in the service ticket MUST be set.
In the key sequence case, the sequence of keys is sent to the change
or set password service (kadmin/changepw or kadmin/setpw
respectively). For a principal that can act as a server, its
preferred keytype should be sent as the first key in the sequence,
but the KDC is not required to honor this preference. Application
servers SHOULD use the key sequence option for changing/setting their
keys. The change/set password services should check that all keys are
in the proper format, returning the KRB5_KPASSWD_MALFORMED error
otherwise.
Trostle, Swift, Brezak, Gossman [Page 7]
INTERNET DRAFT June 2001 Expires December 2001
For change/set key, the request message may include the request flags
bit string with the request-srv-gen-keys bit set. In this case, the
client is requesting that the server add entropy to its keys in the
KeySequences field. When using this option, the client SHOULD attempt
to generate pseudorandom keys with as much entropy as possible in its
request. The server will return the final key sequence in a
KeySequences structure in the edata of the reply message. The server
does not store any of the new keys at this point. The client MUST
make a subsequent change/set key request without the request-srv-
gen-keys bit; if the server returns KRB5_KPASSWD_SUCCESS for this
second request, then the new keys have been written into the
database. A conformant server MUST support this option.
Reply Message
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| message length | protocol version number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| AP-REP length | AP-REP data /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ KRB-PRIV message /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
All 16 bit fields are in network byte order.
message length field: contains the number of bytes in the message
including this field.
protocol version number: contains the hex constant 0x0002 (network
byte order). (The reply message has the same format as in the
original Kerberos change password protocol).
AP-REP length: length of AP-REP data, in bytes. If the length is
zero, then the last field contains a KRB-ERROR message instead of a
KRB-PRIV message. An implementation should check this field to
determine whether a KRB-ERROR message or KRB-PRIV message has been
returned.
AP-REP data: the AP-REP is the response to the AP-REQ in the request
packet. The subkey MUST be present in the AP-REP message.
KRB-PRIV message: This KRB-PRIV message must be encrypted using the
subkey from the AP-REP message. The client should ignore the sender
address (the server's address) in the KRB-PRIV message. Reflection
attacks are prevented since the subkey is used to encrypt the user-
data field of the KRB-PRIV message. The timestamp and usec fields of
Trostle, Swift, Brezak, Gossman [Page 8]
INTERNET DRAFT June 2001 Expires December 2001
the KRB-PRIV message MUST be present, and the data values MUST be
copies of the same data values from the AP-REP message.
The server will respond with a KRB-PRIV message unless it cannot
validate the client AP-REQ or KRB-PRIV message, in which case it will
respond with a KRB-ERROR message.
The user-data component of the KRB-PRIV message, or e-data component
of the KRB-ERROR message, must consist of the following data.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| result code | key version (only on success) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| result string length | result string /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| edata /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
result code (16 bits) (result codes 0-4 are the same as in the
original Kerberos change password protocol):
The result code must have one of the following values (network
byte order):
KRB5_KPASSWD_SUCCESS 0 request succeeds (This
value is not allowed in a
KRB-ERROR message)
KRB5_KPASSWD_MALFORMED 1 request fails due to being
malformed
KRB5_KPASSWD_HARDERROR 2 request fails due to "hard"
error in processing the
request (for example, there
is a resource or other
problem causing the request
to fail)
KRB5_KPASSWD_AUTHERROR 3 request fails due to an
error in authentication
processing
KRB5_KPASSWD_SOFTERROR 4 request fails due to a soft
error in processing the
request
KRB5_KPASSWD_ACCESSDENIED 5 requestor not authorized
KRB5_KPASSWD_BAD_VERSION 6 protocol version unsupported
KRB5_KPASSWD_INITIALFLAG_NEEDED 7 initial flag required
Trostle, Swift, Brezak, Gossman [Page 9]
INTERNET DRAFT June 2001 Expires December 2001
KRB5_KPASSWD_POLICY_REJECT 8 new cleartext password fails
policy; the result string
should include a text
message to be presented to
the user.
KRB5_KPASSWD_WRONG_SRV 9 policy failure: the client
sent change/set key and
should have sent change/set
passwd, or vice-versa.
KRB5_KPASSWD_BAD_PRINCIPAL 10 target principal does not
exist (only in response to
a set password or set key
request).
KRB5_KPASSWD_ETYPE_NOSUPP 11 the request contains a key sequence
containing at least one etype that
is not supported by the KDC. The
response edata contains an ASN.1
DER encoded PKERB-ETYPE-INFO type
that specifies the etypes that the
KDC supports:
KERB-ETYPE-INFO-ENTRY :: = SEQUENCE
{
encryption-type[0] INTEGER,
salt[1] OCTET STRING
OPTIONAL -- not sent, client
-- may ignore if
-- sent
}
PKERB-ETYPE-INFO ::= SEQUENCE OF
KERB-ETYPE-INFO-ENTRY
The client should retry the request
using only etypes (keytypes) that
are contained within the
PKERB-ETYPE-INFO structure in the
previous response.
KRB5_KPASSWD_ETYPE_SRVGENKEYS 12 See the following paragraph.
The KRB5_KPASSWD_ETYPE_SRVGENKEYS result code is returned when
the request has the request-srv-gen-keys flag set and the
server is returning the KeySequences structure defined above in
the edata field of the reply. The server returns one key sequence
structure of the same keytype for each key sequence structure in
the client request, unless it does not support one of the
keytypes (or etypes). In that case, it returns error
KRB5_KPASSWD_ETYPE_NOSUPP as discussed above. The server MUST
add keylength number of bits of entropy to each key, where
Trostle, Swift, Brezak, Gossman [Page 10]
INTERNET DRAFT June 2001 Expires December 2001
keylength is the number of actual key bits in the key (minus
any parity or non-entropy contributing bits). The assumption
here is that the client may have added insufficient entropy
to the request keys. The server SHOULD use the client key from
each KeySequence structure as input into the final keyvalue for
the returned key. The client MUST make another request after
receiving a reply with this status, since no keys have been
written into the database.
0xFFFF is returned if the request fails for some other reason.
The client must interpret any non-zero result code as a failure.
key version (16 bits - optional):
Present if and only if the result
code is KRB5_KPASSWD_SUCCESS. This contains the key version of
the new key(s).
result string length (16 bits):
Gives the length of the following result string field, in bytes.
If the result string is not present, the length is zero.
result string (optional):
This field is a UTF-8 encoded string which can be displayed
to the user. Specific reasons for a password set/change policy
failure is one possible use for this string.
edata (optional):
Used to convey additional information as defined by the
result code.
5. Acknowledgements
The authors thank Ken Raeburn, Tom Yu, Martin Rex, Sam Hartman, Tony
Andrea, Nicolas Williams, and other participants from the IETF
Kerberos Working Group for their input to the document.
6. Security Considerations
Password policies should be enforced to make sure that users do not
pick passwords (for change password/key) that are vulnerable to brute
force password guessing attacks.
7. References
[1] Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
[2] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
[3] J. Kohl, C. Neuman. The Kerberos Network Authentication
Service (V5), Request for Comments 1510.
Trostle, Swift, Brezak, Gossman [Page 11]
INTERNET DRAFT June 2001 Expires December 2001
8. Expiration Date
This draft expires on December 31st, 2001.
9. Authors' Addresses
Jonathan Trostle
Cisco Systems
170 W. Tasman Dr.
San Jose, CA 95134
Email: jtrostle@cisco.com
Mike Swift
University of Washington
Seattle, WA
Email: mikesw@cs.washington.edu
John Brezak
Microsoft
1 Microsoft Way
Redmond, WA 98052
Email: jbrezak@microsoft.com
Bill Gossman
Cisco Systems
500 108th Ave. NE, Suite 500
Bellevue, WA 98004
Email: bgossman@cisco.com
10. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
Trostle, Swift, Brezak, Gossman [Page 12]
INTERNET DRAFT June 2001 Expires December 2001
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
Trostle, Swift, Brezak, Gossman [Page 13]